Due to the increasing interest in the environment and understanding of the ecological cycle in nature, there is a great desire among both consumers and producers to decrease discharges of pollutants arising as a consequence of human activity. Therefore, vigorous efforts have been made to decrease the discharges from pulp and paper mills.
One aim has been to create a closed pulp mill in which there are no discharges from the pulp mill. In a closed pulp mill, the process chemicals are recovered from the spent liquors and reused. A problem associated with a closed pulp mill has been making it possible to return bleaching department spent liquors in countercurrent to the pulp in those cases where the spent liquors contain chlorine chemicals from a bleaching stage, which is based on chlorine gas or chlorine dioxide. Successful attempts have been made to circumvent this problem by avoiding chlorine-containing chemicals, and instead using hydrogen peroxide or ozone as bleaching chemicals.
Another problem has been the risk of certain non-process elements, for example ions of transition metals, which are supplied to the process together with the raw wood material, building up in high concentrations in the system when the spent liquors are returned. Such metal ions are often chelated, especially prior to a sensitive hydrogen peroxide bleaching stage where there is the risk that the peroxide will be decomposed by the metal ions. However, when filtrate from a washing stage which succeeds such a chelating stage is returned in countercurrent, there is the risk that the metal ions will be returned to the pulp by means of precipitating out on the pulp.
An alternative to returning the bleaching department spent liquor in countercurrent to the pulp is to convey the spent liquor to the mixing department where it can be used as washing water, or to the soda smelt dissolver, where it can be used as make-up water. However, the quantity of spent liquor is usually too great for these uses, and, as a result, evaporation is necessary. A problem with this is that the spent liquor from a chlorine gasfree or chlorine dioxide-free bleaching department can contain large quantities of calcium which may precipitate out when the spent liquor is evaporated and form undesirable encrustations on the equipment.
Swedish patent No. 417 114 attempts to solve the encrustation problem by adding a metal compound that is able to chelate encrustationforming negative ions.
Southern Pulp and Paper Manufacture, 40/1977, No.8, pages 16-36, "Evaporator Scaling", T. Grace, discloses a method for "thermally deactivating" calcium in black liquor in order to avoid encrustation formation. This method consists of heat-treating the black liquor, to which calcium carbonate has been added to act as crystallization nuclei, at 150.degree. C. for 10-15 minutes. The theory being the method is that the calcium in the black liquor is bound to organic substances, for example dissolved lignin and oxalate ions. During the heat treatment, the complex is broken down and the calcium ions precipitate out due to reaction with the carbonate ions which are naturally present in the black liquor. Calcium carbonate in solid (precipitated) form is not regarded as a cause of encrustation formation when liquid which contains such a precipitate is evaporated. Encrustations are only formed if the calcium carbonate precipitates out directly onto the hot heat-transfer surfaces of the evaporator. This method has been found to be successful in preventing encustation but cannot be directly applied to the bleaching department spent liquors since the neutral content of carbonate ions is low.
Thus, there is a need for a method of reducing encrustations on the evaporator equipment caused by evaporating a bleaching plant spent liquor containing calcium.